: Herpes simplex virus (HSV) encodes multiple genes which inhibit apoptosis of infected cells. Viruses which inhibit apoptosis may have a replicative advantage, by ensuring that the host cell lives until viral replication is complete. One mechanism by which the host attempts to induce apoptosis of infected cells is through the action of cytotoxic T lymphocytes (Cm). CTh function is critical for the control of HSV infections, and patients with impaired CU function can have severe HSV recurrences and delayed healing. Therefore, HSV has evolved mechanisms which blunt the attack of CU, such as downregulation of MHC class I via 1CP47. We recently demonstrated a second mechanism used by HSV to blunt the attack of CU, in which HSV protects infected cells from CTL-induced DNA fragmentation. However, inhibition of DNA fragmentation may not imply inhibition of other aspects of cell death (cell lysis, caspase activation, etc.). Furthermore, CU trigger different death pathways in their target cell, depending on the CU effector mechanism used. Therefore, we hypothesize that while HSV inhibits DNA fragmentation, other aspects of cell death induced by CU may remain unaffected. Furthermore, we hypothesize that not all measures of cell death will correlate equally well with the prevention of viral replication in the infected cell. The aims of this project, therefore, are to evaluate the effects of HSV and its anti-apoptosis genes on cell death, after induction of death by CTL or isolated CTL effector mechanisms. The data generated may ultimately allow rational immune modulatory approaches to HSV disease. Specific Aim 1. Determine whether HSV protects infected cells from death-receptor dependent mechanisms of killing by CTL. Our preliminary data demonstrate that HSV infection protects infected LCL or Jurkat cells from apoptosis triggered through the fas receptor. We will therefore test whether HSV or its individual anti-apoptosis genes protect cells from death induced by fas ligation, TNF, or TRAIL, as measured by cell morphology, DNA fragmentation, 51Cr release, caspase activation, mitochondrial transmembrane potential, clonogenicity of infected cells, and viral yield. Specific Aim 2. Determine whether HSV protects infected cells from lytic-granule dependent mechanisms of killing by CTL. Granule-mediated cytotoxicity appears to be the predominant mechanism for CTL killing of nonhematopoietic cells, and thus protection from this may be critical for effective immune evasion. Specific Aim 3. Determine whether HSV protects infected cells from damage and death induced by whole CTL. Protection from whole CTL may most closely reflect the in vivo situation. The data generated in this aim will provide the first comprehensive evaluation of the ramifications of CTL attack on the various aspects of fibroblast death, and will provide an understanding of the protective effect of HSV infection on each of these parameters.